Fluids which exhibit significant change in their properties of flow in the presence of an electric field have been known for several decades. Such fluids were first referred to as "electroviscous" because their apparent viscosity changes in the presence of electric fields. As understanding of these types of fluids has grown, it has now become apparent that the phenomena being observed is a change in the minimum stress required to induce shear in the fluid, while the actual viscosity may remain generally constant. Accordingly, these effects are better understood in terms of the total rheology of the fluids and such compositions are now more commonly referred to as "electrorheological" ("ER") fluids.
Early studies of electrorheological fluids were performed by W. M. Winslow, some of which are reported in U.S. Pat. Nos. 2,417,850 and 3,047,507. Winslow demonstrated that certain suspensions of solids (the "discrete," "dispersed" or "discontinuous" phase) in liquids (the "continuous" phase) show large, reversible electrorheological effects. These effects are generally as follows: In the absence of an electric field, electrorheological fluids exhibit Newtonian behavior; specifically, their shear stress (applied force per unit area) is directly proportioned to the shear rate (relative velocity per unit thickness). When an electric field is applied, a yield stress phenomenom appears and no shearing takes place until the shear stress exceeds a yield value which rises with increasing electric field strength. This phenomenon can appear as an increase in apparent viscosity of several, and indeed many, orders of magnitude.
In laymen's terms, an ER fluid initially appears as a liquid which, when an electric field is applied, acts almost as if it had become a solid.
Electrorheological fluids change their characteristics very rapidly when electric fields are applied or released, with typical response times being on the order of one millisecond. The ability of ER fluids to respond rapidly to electrical signals gives them unique characteristics as elements in mechanical devices. Often, the frequency range of a mechanical device can be greatly expanded by using an ER fluid element rather than an electromechanical element having a response time which is limited by the inertia of moving mechanical parts. Therefore, electrorheological fluids offer important advantages in a variety of mechanical systems, particularly those which require a rapid response interface between electronic controls and mechanical devices.
All sorts of devices have been proposed to take advantage of the electrorheological effect. Because of their potential for providing a rapid response interface between electronic controls and mechanical devices, these fluids have been applied to a variety of mechanical systems such as electromechanical clutches, fluid filled engine mounts, high speed valves with no moving parts, and active dampers for vibration control among others.
A rather wide variety of combinations of liquids and suspended solids can demonstrate electrorheological effects. As presently best theorized, the basic requirements for an ER fluid are fine dielectric particles, the surface of which typically contains adsorbed water or some other surfactant or both, suspended in a non-polar dielectric fluid having a permittivity less than that of the particle and a high breakdown strength. As used herein, the term "dielectric" refers to substances having very low electrical conductivities. Such substances have conductivities of less than 1.times.10.sup.-6 mho per centimeter. These are rather general requirements, and accordingly a wide variety of systems have been found to demonstrate ER effects. Winslow's initial work was performed using materials as simple as starch in mineral oil. As analysis of these materials has continued, other materials have been investigated, with common ones being silica and silicone oils as the discrete and continuous phases, respectively.
There are a number of proposed hypotheses for explaining the mechanism through which electrorheological fluids exhibit their particular behavior. All of these center around the observation that the electrorheological effect appears in suspensions in which the permittivity of the discrete phase particles is greater than that of the continuous phase. A first theory is that the applied electric field restricts the freedom of particles to rotate, thus changing their bulk behavior. A second theory describes the change in properties to the formation of filament-like aggregates which form along the lines of the applied electric field. One present theory proposes that this "induced fibration" results from small lateral migrations of particles to regions of high field intensity between gaps of incomplete chains of particles, followed by mutual attraction of the particles.
A third theory refers to the "electric double layer" in which the effect is explained by hypothesizing that the application of an electric field causes a layer of materials adsorbed upon the discrete phase particles to move, relative to the particles, in a direction along the field toward the electrode having a charge opposite that of the mobile ions in the adsorbed layer. As used herein, the term adsorption refers to the adherence of the atoms, ions or molecules of a gas or liquid to the surface of another substance which is referred to as the adsorbent. This differs from absorption which refers to the penetration of one substance into the inner structure of another.
Yet another theory proposes that the electric field drives water to the surface of the discrete phase particles through a process of electro-osmosis. The resulting water film on the particles then acts as a glue which holds the particles together.
As demonstrated by this wide variety of proposed theories, there exists no single clear cut explanation of all of the observed phenomena. Nevertheless, a number of empirical parameters have been identified which tend to increase or decrease the electrorheological effect in any given fluid. These can be briefly summarized as follows:
Particle size and concentration: In general, higher volume fractions of the dispersed phase afford higher induced yield stresses at constant field strength and shear rate conditions. Some researchers have found it advantageous to use smaller particles, while others have argued that a distribution of particle sizes is desirable. Yet another has concluded that electrorheological effects of a fluid will increase with an increase in particle diameter until a certain size is reached which maximizes the effect, after which a further increase in the size of the particles causes a decrease in the effect. Alternatively, for a given size particle, the electrorheological effects of the fluid will increase linearly with concentration of particles until a maximum value is reached, after which the effect again begins to fall off.
Particle porosity and adsorbed moisture: Some researchers have postulated that the dispersed particles should be sufficiently porous to be capable of adsorbing at least 10 percent by weight of water, and that the adsorption of water on the particles is a prerequisite to the electrorheological effect in a fluid. Although it has been determined that adsorbed water is not always a prerequisite for the electrorheological effect, adsorbed water does have a marked effect on producing electrorheological effects in a great many cases. Overly large amounts of water, however, increase the electrical conductivity of electrorheological fluids and the resulting amount of current required to produce the effect increases exponentially with an increase in water content.
Surface activators and surfactants: In many electrorheological fluids, suspension stabilizers such as surface activators or surfactants demonstrate an increase in the electrorheological response of the fluid, or assist in keeping the solid particles from settling, or both.
Field strength: Electrorheological effects increase with increasing field strength. In studying applied fields, it has been determined that constant applied field strengths at different electrode spacings result in about the same electrorheological behavior, demonstrating that the electrorheological properties of a given fluid are bulk properties of the system, rather than "wall effects" or other geometric factors.
Temperature: The viscosity of electrorheological fluids has been observed to increase with increasing temperature under an electric field, and under a given set of conditions the relative viscosity is higher at higher temperatures. The resistivity of electrorheological fluids, however, has been found to decrease as temperature increases. For example, in water-activated systems the current which will be passed by an electrorheological fluid at a fixed voltage field generally doubles for each rise in temperature of 6.degree. C.
Shear rate: The shear stress of electrorheological fluids increases slightly with shear rate, but not as quickly as shear stress rises in the absence of a field. Accordingly, the "electroviscosity" (the arithmetic difference between apparent viscosity and viscosity in the absence of a field) decreases with increasing shear rate.
A large number of other factors can be shown to have greater or lesser effects on the behavior and response of electrorheological fluids. The basic relationships, however, can be summarized as follows: when only one parameter is varied, electrorheological effects increase with an increasing volume fraction of the dispersed phase, with an increase in field strength, and with an increase in temperature. The effects decrease with increasing shear rate.
Turning to more specific applications, in order to fulfill their potential as a unique interface between electronic controls and mechanical systems, appropriate electrorheological fluids must demonstrate certain practical characteristics. For example, for certain applications an ER fluid should be able to withstand relatively high operating temperatures. Under other circumstances, low power consumption is important. In yet other circumstances, the dispersed phase particles must be non-abrasive. In other circumstances, the dispersed phase must remain dispersed even where some sort of dispersing agitation cannot be provided. As would be expected, the chemical nature of the continuous liquid, the dispersed solid, and any resulting combination should be compatible with the mechanical materials used to produce the electrorheological device.
Many electrorheological devices are more desirably operated at relatively high operating temperatures and low electric field strengths. Such conditions can be less suitable for inducing the electrorheological effect in fluids which rely on water adsorption as part of their electrorheological mechanism, because of the thermal and electrical properties of water. Nevertheless, any electrorheological fluid used in such devices must still demonstrate sufficient electrorheological capabilities as to be useful.
Therefore, there exists a present need for ER fluids which are suitable for use under high temperature and low current conditions, i.e. a material with an appropriately low conductivity, and yet which are physically, mechanically, and chemically compatable with applied systems.
Several systems have already been proposed. Chertkova et al, Kolloidnyi Zhurnal, Vol. 44, No. 1, pp. 83-90, Jan-Feb 1982, discuss the electrorheological behavior of titanium dioxide (TiO.sub.2) dispersions in dielectric fluids to which ten different surfactants were added, but from which water was absent. Because TiO.sub.2 is a semiconductor, however, ER fluids produced according to Chertkova's description could require higher current usage than is desirable for many practical applications.
Makatun et al, Inzh.-Fiz. Zh., 45, 4, 597-602 (1983) (available as library translation 2125 from the Royal Aircraft Establishment) discuss the behavior of several ER fluids, using aluminum dihydrotripolyphosphate (H.sub.2 AlP.sub.3 O.sub.10.2H.sub.2 O) as a primary example for the dispersed particulate phase. Although Makatun does not discuss adsorbed water as being necessary to such systems, he reports that the hydrated character of the compound contributes to the ER effect. Therefore, because H.sub.2 Al.sub.3 O.sub.10.2H.sub.2 O will dehydrate at temperatures of about 130.degree. C. and above, Makatun's compositions would be expected to lose their ER effectiveness in applications taking place at such temperatures.
In another example, Block and Kelly (U.K. Patent Application GB No. 2 170 510 A, Aug. 6, 1986) describe an ER fluid which is effective using an anhydrous dispersed phase. Block and Kelly recognize some of the disadvantages of water-activated ER fluids, but like Cherthova et al suggest that semiconductors--and preferably organic semiconductors--be used as the dispersed phase material. The materials they suggest are generally pigments and tend to form messy fluids which are difficult to handle. Additionally, because the dispersed phase materials are semiconductors, the current densities and power consumption required by the Block and Kelly fluids can be as high as in water-activated systems. This, of course, makes the use of such materials disadvantageous, if not impossible, in applications calling for low current density.
Accordingly, it is an object of the present invention to provide an electrorheological fluid which will demonstrate appropriate electrorheological capabilities in the absence of water.
It is another object of the present invention to provide an electrorheological fluid which exhibits appropriate capabilities in the absence of water and at relatively low current densities.
It is a further object of this invention to provide an improved electrorheological fluid in which the dispersed phase is sufficiently polarizable to give rise to the electrorheological effect, while having a sufficiently low conductivity to prevent electric discharge or excessive current densities while in use.
It is a further object of this invention to provide an electrorheological fluid in which the dispersed phase is a hyperprotonic conductor.
It is another object of this invention to provide an electrorheological fluid in which the properties of polarizability and low conductivity are provided by a dispersed phase solid crystalline material which conducts electricity favorably along only one of the three crystal axes.
It is a further object of the invention to provide a method of preparing an electrorheological fluid which is effective at low current densities and in the absence of adsorbed water or water of hydration by admixing a dielectric liquid with a particulate phase formed from a crystalline material which conducts current only along one of the three crystal axes to form a suspension of the crystalline material in the dielectric liquid.
The foregoing and other objects, advantages and features of the invention, and the manner in which the same are accomplished will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein: